Getting your head around public key encryption isn't easy, even if you're a dab hand at math. Prime factors? Modular arithmetic? Say what? Instead, here's a totally straightforward explanation that starts from the basics, using paint mixing as an example.

No, really! The key to understanding public key encryption is realising that it's easy to combine two things—like mixing paint or multiplying big numbers together—but it's hellish difficult to reverse the process. But, I won't explain any further when someone else has done such a perfect job of doing it already. Enjoy. [YouTube]